Disc brake squeal is a high pitch sound/vibration originating from a vehicle's braking system. The squeal may occur frequently depending on car make, brake design and weather conditions. The trend by car builders is to use less weight in the brake and the body of the car which aggrevates the occurrence of brake noise.
Noise problems in braking systems have occurred since the automobile came into existence. In the early days there were so many other noises from a car that brake noise was never considered a problem. In the late 1960's and early 70's car manufacturers started using disc brakes drastically decreasing the stopping distances. Initially, the majority of the brake pads contained asbestos as a main component, which resulted in little brake noise problems. Today essentially all major car manufacturers world wide have eliminated the use of asbestos following legislation of environmental issues. Instead, most manufacturers are using semi-metallic friction materials in their brake pads. This metal-to-metal contact with the brake disc has often increased the noise generation.
In order to reduce or eliminate brake squeal, today, nearly all the cars being manufactured around the world contain brake pads equipped with anti-noise shims/insulators/dampers to eliminate brake squeal.
The squeal-noise generated by disc brakes is a vibration that is the result of the brake assembly parts vibrating at high frequency. Studies show that squeal can occur from a number of possible sources. The most common sources are: brake caliper components, friction material of brake pads and disc rotors.
To reduce or eliminate brake squeal noise, anti-squeal shims are provided in between the caliper and the backing plate of the brake pads. Such anti-squeal shims are conventionally comprised of a steel plate with damping material layers applied on both faces thereof In most cases, the damping material layers of the anti-squeal shims is formed of rubber and selected to be about 100 μm thick. The Japanese Patent Publication No. Showa 63-101530, discloses an anti-squeal shim is wherein the damping material layers are formed of a compound including a nonmetal fibre other than asbestos, an elastomer and a filler. The fibre reinforced damping material layers are about 100 μm, and are shown to have improved wear resistance compared to non reinforced rubber. U.S. Pat. No. 6,105,736 discloses an anti-squeal shim comprising a metal plate and a fibre reinforced compound layer applied only to the brake pad side of the metal plate whereas the piston side of the metal plate is in direct contact with the piston. The fibre reinforced compound includes a nonmetal fibre other than asbestos, an elastomer and a filler. The thickness of the compound layer is determined to be from 200 to 800 micron. The disclosed anti-squeal shim is disposed between the brake piston and the backing plate of the friction pad with the metal plate opposed to the piston without the intervention of a backing-up shim and with the compound layer opposed to the backing plate. In U.S. Pat. No. 6,105,736 it is stated that when the compound layer is thinner than 200 micron, satisfactory vibration damping properties can not be obtained, while when it is thicker than 800 micron, a stress relaxation is liable to occur in the anti-squeal shim.